1. Field of the Invention
The present invention relates to the diagnostics of fiber optic data links and more particularly to utilizing an existing photodiode and laser for performing the diagnostics.
2. Description of the Prior Art
There are several techniques for monitoring fiber optic links. One such technique is described in U.S. Pat. No. 5,515,361 entitled xe2x80x9cLink Monitoring And Management In Optical Star Networksxe2x80x9d issued on May 7, 1996 and assigned to the same assignee as the present invention. This patent describes that when one link of a pair of links fails, the failure is detected by a monitor. When the pair of links again become operable, a signal is detected as passing from the transmitter through a loop and back to the receiver. U.S. Pat. No. 5,673,132 entitled xe2x80x9cApparatus For Repowering And Monitoring Serial Linksxe2x80x9d, issued on Sep. 30, 1997 and assigned to the same assignee as the present invention, describes a computer system that employs a repeater unit that monitors and records non-idle usage and errors for both directions of a repeated serial link. The system can also send certain diagnostic patterns to assist with link determinations.
U.S. Pat. No. 5,488,621 entitled xe2x80x9cLaser Bias And Modulation Circuitxe2x80x9d issued on Jan. 30, 1996, and U.S. Pat. No. 5,402,433 entitled xe2x80x9cApparatus And Method For Laser Bias And Modulation Controlxe2x80x9d issued on Mar. 28, 1995, both describe systems for adjusting the bias current and the modulation current amplitude of the laser diode. The use of monitor photodiodes is included in the description.
An article entitled xe2x80x9cOptical Time-domain Reflectometer With A Semiconductor Laser Amplifierxe2x80x9d by K. Suzuki, T. Horiguchi and S. Seikai, Electronic Letters, Vol. 20, No. 18, pages 714-716, Aug. 30, 1984, describes using a semiconductor laser as an optical pulse generator and as a backscattered optical signal amplifier. The system receives analog backscatter data and uses digital signal processing to average and display the analog data. The system specifically avoids obtaining the reflected pulse from the end of the fiber by dipping the fiber end in index matching fluid. Also, the photodiode is not used to control the laser bias and the laser is not used for data communication. This system is also limited to longwave and single mode fiber.
It is an object of the present invention to provide a fiber optic data link diagnostic system that utilizes typical parts of an existing fiber optic transmitter system. It is a further object to provide a system that receives digital data (a reflected pulse) and uses digital signal processing to locate the reflected pulse.
The present invention utilizes a monitor photodiode and a laser to perform diagnostic measurements on a fiber optic data link. The monitor photodiode is typically part of the laser diode package and is normally used for monitoring the bias point of the laser. The laser is normally only used for transmitting data. The present invention receives digital data (a reflected pulse) and uses digital signal processing to locate the reflected pulse and to use the round trip time of the reflected pulse to determine a fault location or the link length.
The present invention includes a diagnostic system connected to a fiber optic transceiver. The diagnostic system includes a multiplexer connected between the encoder and the laser driver of the fiber optic data link transceiver. The multiplexer is controlled by a normal op/test mode selector which also enables a test controller. The multiplexer also receives a pulse or pattern for the test mode from a pulse/pattern generator. A signal from the monitor photodiode flows into a pulse/pattern detection circuit which signals the test controller and a register if a pattern is found. The test controller also controls a laser bias adjust circuit which modifies the inputs to the laser driver. The diagnostic system also includes a counter connected to the register which captures the count after a reflected pulse is detected and forwards the count to the service processor to be analyzed.